1. Field of the Invention
The present invention relates to a transverse magnetic (TM) mode dielectric resonator and to a TM mode dielectric filter and duplexer using the resonator.
2. Description of the Related Art
A known dielectric filter using a TM mode dielectric resonator is shown in FIG. 13. Each of the dielectric resonators shown in FIG. 13 is a dual mode type comprising a plurality of dielectric blocks of short-circuit type TM.sub.110 mode dielectric resonators which are integrally combined in a crisscross fashion. This structure enables each TM mode dielectric resonator to have the function of two TM mode dielectric resonators while being equal in size to one ordinary dielectric resonator of this kind.
Referring to FIG. 13, a dielectric filter 101 has four TM dual mode dielectric resonators 102, 103, 104, and 105, which are arranged in a row in respective cavity casings, with the openings defined by the respective cavity casings facing in the same direction. Metallic panels 106 and 107 are attached to these dielectric resonators so as to cover the openings.
The TM dual mode dielectric resonator 102 has a cavity casing 102a having openings on the front and rear sides as viewed in FIG. 13, and a dielectric crisscross block 102XY. The cavity casing 102a and the dielectric crisscross block 102XY are integrally formed of the same dielectric material. A conductor 102b is formed on the outer surface of the cavity casing 102a except on the front and rear opening edges. The cavity casing l02a with the conductor 102b forms a shielded cavity. The dielectric block 102XY is formed of a horizontal portion 102X and a vertical portion 102Y as viewed in FIG. 13. Thus, the TM dual mode dielectric resonator 102 forms a two-stage resonator. Each of the TM dual mode dielectric resonators 103, 104, and 105 has the same structure as the TM dual mode dielectric resonator 102.
An input loop 108 and an output loop 109 are mounted on the panel 106. The input loop 108 and the output loop 109 are connected to external circuits via coaxial connectors (not shown).
Coupling loops 107a, 107b, 107c, and 107d for coupling each adjacent pair of the TM dual mode dielectric resonators are mounted on the panel 107.
In dielectric resonators for use in such a dielectric filter, the resonant frequency of each dielectric resonator is determined by the size of the cavity and the size of the dielectric block.
For example, in the case of an ordinary TM.sub.110 mode dielectric resonator having a single vertical dielectric block structure, the resonant frequency becomes lower if the width of the cavity is increased while the width, thickness and height of the dielectric block and the height of the cavity are fixed. The resonant frequency becomes lower if the width or thickness of the dielectric block is increased while the size of the cavity is fixed. Also, when the frequency is fixed, an increase in the unloaded Q of the dielectric resonator is attained by increasing the height of the dielectric block.
In such a case, if the height of the dielectric block is increased, the height of the cavity is necessarily increased. But since a real current flows through the conductor on the cavity casing surface in the TM.sub.110 mode dielectric resonator, the loss in the conductor on the cavity casing surface becomes larger when the size of the cavity casing is increased. However, the increase in unloaded Q achieved by enlarging the cavity is sufficiently large to compensate for the loss in the conductor on the cavity casing surface. Consequently, the unloaded Q becomes higher when the height of the dielectric block is increased.
If the loss in the conductor on the cavity casing surface can be reduced, the unloaded Q can be further increased while limiting the increase in the height of the dielectric block. Therefore, there has been a need for a dielectric resonator designed to have reduced loss in the conductor on the cavity casing surface.
In the TM dual mode dielectric resonator shown in FIG. 13, when the sizes of the vertical and horizontal portions of the dielectric block are adjusted to obtain a predetermined frequency, the size of the cavity is also affected. To increase the unloaded Q, therefore, it is necessary to increase both the width and height of the cavity, resulting in an increase in the overall size of the dielectric filter. Also, the resonant frequency becomes lower if the cavity size is increased while the size of the dielectric block is fixed. Therefore, if the size of the cavity is increased, the width or thickness of the dielectric block is necessarily reduced. Thus, in the conventional TM dual mode dielectric resonator, it is difficult to independently change both the unloaded Q and the frequency.